The skin is an organ of the body that performs vital functions including protection against external physical, chemical, and biologic aggressions. The skin is made up of three layers: the epidermis, the dermis, and hypodermis. The epidermis is the outermost part of the skin. The middle layer, the dermis, is basically made up of the fibrous structural protein called collagen, and it is a connective tissue of fibroblasts and an extracellular matrix that functions to provide cohesion and nutrition to the skin. The dermis lies on the hypodermis, which includes adipocytes, connective tissue, and elastins, providing an energy reservoir and thermal protection.
The epidermis includes several cell layers composed primarily of keratinocytes. The epidermis harbors other cell populations such as dendritic cells, melanocytes, Langerhans cells, and Merkel cells, but keratinocytes by far make up the majority of the cells. The epidermis is commonly divided into four layers: the basal layer (stratum germinativum), the spinous layer (stratum spinosum), the granular layer (stratum granulosum), and the cornified or horny layer (stratum corneum).
Human skin is continually being renewed. The outermost cells of the horny layer are continually desquamated and replaced by cells of a lower layer by a cellular process of epidermis renewal undertaken by keratinocytes. The cells that form at the basal layer through mitosis progressively differentiate and migrate (move up) through the layers of the skin to reach the horny layer. As the cells move up the strata, they generate keratin, a fibrous protein that is water insoluble, and which progressively fills the cells. As they move towards the upper layers, the keratinocytes degenerate and secrete a cement, which increases cohesion between the cells, thereby effectively creating a waterproof barrier over the body's surface.
Skin aging follows a normal process of senescence which may be exacerbated by external factors such as exposure to ultraviolet radiation from the sun. With aging, the epidermis thins, even though the number of cell layers may remain unchanged. Large pigmented spots may appear in sun-exposed areas. Changes in the connective tissue reduce the skin's strength and elasticity.
Aesthetically, these changes in the skin due to aging or external factors result in a change in the appearance of the skin. The skin may appear rough or less smooth, may develop more lines or wrinkles, and/or may show stretch marks and redness. Additionally, the skin may lose elasticity and firmness, and/or skin complexion may appear less luminous.
Many existing cosmetic products on the market purport to prevent or reduce wrinkles, redness, or stretch marks. However, many of these products entail lengthy waiting periods before visible results are achieved, are simply less effective than claimed, or not effective at all. Additionally, to achieve effective anti-aging performance, a regimen of several different skincare products is typically required.